It is known that one can open a tank vent valve during the operation of the engine and evaluate a reaction from a fuel/air ratio control loop for the diagnosis. The fuel vapor mixed with air, from the tank venting (purge gas) has the effect of disturbing the control circuit, so that the occurring of the disturbance indicates an efficient tank venting, and thus particularly a functional tank venting valve.
The problem is that, in engines that are able to run on lean mixtures, the reaction turns out to be weaker, the lower the proportion of fuel in the purge gas. In the case of purge gas that is loaded only weakly with fuel, no reliable differentiation between defective and functional systems is possible.
The present invention aims at making possible a reliable diagnosis that is not dependent on the fuel proportion of the purge gas.
This aim is achieved using a method for testing the operability of a tank venting valve between an internal combustion engine and a fuel vapor trap, the stored fuel vapor being guided from the fuel vapor trap to the internal combustion engine when the tank vent valve is open, and the fuel vapor supply representing a first energy flow to the internal combustion engine, and air also flowing to the internal combustion engine via a throttle valve, and a second energy flow being assigned to this air, and means being provided which maintain the sum of the two energy flows at a predefined value when the tank vent valve is controlled in an opening manner, and the tank vent valve being controlled in an opening manner, and a change in the energy flow delta E via the throttle valve, resulting from the opening control, being determined and being compared to a predetermined threshold, and a small change in the energy flow, which does not exceed the threshold value (threshold) being evaluated as a fault in the tank vent valve.
A further specific embodiment provides that a sufficiently great energy flow change, which exceeds the threshold value (threshold), is evaluated as a sign of a well functioning tank vent valve.
A further specific embodiment provides that the energy flow through the throttle valve is defined as the product of the air flowing through the throttle valve and the efficiency with which this air is burned after being mixed with fuel.
Another specific embodiment provides that, for determining the energy flow change, first of all a first charge detection is made via an intake manifold pressure sensor, and a second charge detection is made via an evaluation of the throttle valve position in conjunction with the engine speed, and the cylinder charge with air at a given engine speed being determined by the partial pressure proportion of the air in the intake manifold pressure, and the air mass flowing through the throttle valve, which represents a factor of the energy flow, being controlled by the control device in such a way that, for example, at constant load of the engine when idling a stable engine speed sets in.
Still another specific embodiment provides that, when the throttle valve is already almost completely closed even before the opening of the tank vent valve, the additional torque which results from the additional charge from the open tank vent valve, is dissipated via a decline in ignition timing efficiency.
The present invention also relates to an electronic control device for implementing at least one of the methods and specific embodiments mentioned above.
What is essential is an opening of the tank vent valve and an evaluation of the change, resulting from the opening, of a quantity which may be denoted, so to speak, as the energy flow through the throttle valve. In this context, the energy flow may be defined as the product of the air flowing through the throttle valve and the efficiency with which this air is burned after being mixed with fuel.
By contrast to an energy flow change that is too low, a sufficiently great energy flow change, which exceeds the threshold value (threshold), may be evaluated as a sign of a well functioning tank vent valve.
For determining the energy flow change, first of all a first charge detection is made via an intake manifold pressure sensor, and a second charge detection is made via an evaluation of the throttle valve position in conjunction with the engine speed. In this context, the cylinder charge with air at a given engine speed may be determined by the partial pressure proportion of the air in the intake manifold pressure, and the air mass flowing through the throttle valve, which represents a factor of the energy flow, may be controlled by the control unit in such a way that, for example, at constant load of the engine when idling, a stable engine speed sets in. In an efficient overall system, the intake manifold pressure does not change when the tank vent valve is opened, because the additional opening cross section of the tank vent valve is compensated for by a reduction in the opening cross section of the throttle valve. The reduction in the opening cross section of the throttle valve is correlated with the change in the energy flow through the throttle valve.
In one specific embodiment, when the throttle valve is already almost completely closed even before the opening of the tank vent valve, there then occurs a dissipation of the additional torque which results from the additional charge from the open tank vent valve, via a decline in ignition timing efficiency. In other words: If no sufficient compensation for the purge gas influence is possible by resetting the throttle valve opening angle, the resulting additional torque is reduced by an efficiency decline in another parameter.
The method according to the present invention advantageously permits a reliable differentiation between defective and well functioning tank vent valves, independently of the fuel proportion in the purge gas.
In particular it advantageously permits a diagnosis of the tank vent valve in lean operation, as is meaningful, for instance, in the case of internal combustion engines having direct gasoline injection. Because the diagnosis may be carried out in lean operation, it is not necessary to interrupt the lean operation for a diagnosis of the tank vent valve. This saves fuel compared to a diagnosis outside the lean operation.